The 3GPP V2X resource allocation framework offers two entity classes for coordinating vehicle communication --- base-station scheduling and autonomous vehicle selection --- a design space we show to be structurally incomplete for safety-critical coordination. We propose Mode 0, a third entity class centered on the Roadside Computing Unit (RCU): infrastructure integrating elevated sensing, sidelink communication, and local computation, owned by traffic management authorities rather than network operators. We formalize Mode 0's subfamily taxonomy (Mode 0a through 0c), its traffic risk taxonomy and mandatory-escalation boundary, its cross-domain coexistence with Modes 1-4, and its standardization pathway. Of the three failure modes motivating this proposal --- scheduling saturation, occlusion-driven information gaps, and an institutional authority gap during severe hazards --- only scheduling is evaluated by simulation; information and authority are argued architecturally and evidenced by independent deployment convergence across Chinese national standards, Chinese operator infrastructure, and European and US C-V2X programs, properties no channel-level simulation can meaningfully instantiate. A fifteen-run multi-agent reinforcement learning simulation shows shared-actor policies empirically converge to the analytical coordination floor for independent uniform selection. Pairing per-vehicle actors with demand separation achieves strict Pareto improvement for both traffic classes when the safety pool is correctly sized, and --- in the undersized regime where collision-free assignment is combinatorially impossible --- still lifts worst-TTI delivery reliability (5th-percentile intra-episode M0 packet delivery ratio) from 0.113 to 0.601, the only tested configuration whose delivery protection holds structurally rather than on average. We call for a 3GPP study item to formalize Mode 0.

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